A. Field of the Invention
The present invention relates to a method and apparatus for assuring that an automatic implantable cardioverter/defibrillator ("AICD") provides the necessary shock energy through a preselected combination of electrodes.
B. Description of the Related Art
AICDs deliver shock energy through a combination of electrodes. For example, AICD housings (cans) have been developed which allow an external programmer to enable or disable these electrodes. At implant, the physician may select a combination of electrodes which produces the lowest defibrillation threshold (DFT) or which is the most expedient and least traumatic to the patient. If after implant, one of the electrodes breaks, dislodges or shows a rise in impedance, the likelihood of successful defibrillation or cardioversion procedure may decrease significantly.
In prior art approaches, in order to restore the likelihood of defibrillation or cardioversion, the physician had to diagnose the electrode impedance problem and either implant a new electrode or externally reprogram the AICD to use the remaining functional electrodes. The delay between onset of the impedance problem and the corrective action posed a severe hazard to the patient if arrhythmias occurred during this period.
Lead related problems are not uncommon in AICDs. See, Magney et al., PACE 16:445-457 (1993). These failures may arise due to anatomical mechanisms such as failure of central venous catheters adjacent to the sterno-clavicular joint. Problems with AICD leads, such as lead fracture or loose connections, may result in sudden death if ventricular arrhythmias are not detected or not terminated by the AICD. See, Feldman et al., "Identification of An Implantable Defibrillator Lead Fracture With a New Holter System," PACE 16:1342-1344 (1993).
In one study, one hundred and fourteen patients undergoing successful implantation of an AICD were monitored. See, "Lead-Related Morbidity in Patients With Cardioverder-Defibrillators Utilizing Non-Thoracotomy Lead Systems," Nallamothu, N. et al., PACE Vol. 17 No. 4, Part 2, page 761 (NASPE 1994 Abstracts). Twenty-two patients (19%) experienced after implantation lead-associated events which led to re-operation, re-hospitalization, and/or prolongation of implant hospital stay for alteration of anti-arrhythmic drug therapy. The causation of the lead associated events which led to the alternative therapies included lead dislodgement, lead fracture/malfunction, unacceptable DFT in the followup, and subclavian thrombosis. The study proposed that the calculated incident of failure might be an underestimate, as some devices do not yield information on the lead integrity in routine followup and it is not routine to repeat defibrillation efficacy testing on a long term basis. See also, Troop, P. J., "Implantable Cardioverters and Defibrillators," In Current Problems in Cardiology. Yearbook Medical Publishers, pp. 673-815 (1989).
It is known that monitoring impedance of AICD systems is one means for detecting lead failures. See, "Troubleshooting Implantable Cardioverter Defibrillator System Malfunctions: The Role of Impedance Measurements," Haddad, L. et at., PACE Vol. 17 1456-1461 (1994). This study found that external high impedance measurements may be used to troubleshoot AICD system malfunctions due to: lead fractures, faulty adapter connections, and loose set screw connections. However, prior art approaches such as the ones suggested by this study require relatively immediate re-operation to replace the damaged lead.
Routine posterior-anterior and lateral chest x-rays can identify lead fractures prior to any clinical observation. However, other of the cases demonstrate that not all lead or connector problems can be detected by routine chest x-rays.
Certain implantable pacing devices possess a mechanism for switching from a bipolar lead configuration to a unipolar lead configuration when a fault is detected in the bipolar lead ring. See, U.S. Pat. No. 4,964,407. In certain of these devices, a series of high impedance test signals are actively generated (not merely sensed) by a microprocessor in a pacemaker to determine whether a bipolar or unipolar lead is attached. If no operational bipolar lead is detected, the programming of the pacemaker is altered to a unipolar pacing mode. However, similar systems are not available for AICDs.
Methods, circuits and devices are needed which are capable of monitoring the impedance of AICDs for purposes of detecting lead malfunctions. Such systems will avoid the difficulties associated with prior art approaches which require frequent x-raying, inefficient discovery of lead malfunctions using x-raying, or other external monitoring devices. Methods, circuits and devices are also needed which, after detection of a lead malfunction, are capable of connecting or disconnecting leads and electrodes to maximize the likelihood of effective cardioversion and defibrillation. Preferably, such systems will be fully implantable and will not require immediate invasive surgical techniques or frequent x-irradiation in order to correct the problems.